The present invention relates to an apparatus and method for completely incinerating oxidizable impurities in a process gas, including a nonflammable gas stream. It is particularly suitable for treatment of waste gases from semiconductor fabrication processes, and other gases containing highly toxic, pyrophoric, or otherwise undesirable impurities.
Semiconductor manufacturing processes utilize a variety of chemicals, many of which have extremely low human tolerance levels. Such materials include gaseous hydrides of antimony, arsenic, boron, germanium, nitrogen, phosphorous, silicon, selenium and other chemical elements. Typically used compounds include arsine, diborane, germane, hydrogen selenide, phosphine, silane, stibine and many others including halogenated derivatives of the above hydrides, hydrogen chloride, tungsten hexafluoride and a wide variety of metal-organic compounds as well. Acceptable exposure levels for certain of these materials are as low as 50 parts per billion in air that may be inhaled. A significant problem has been the removal of these materials from the effluent gas streams of semiconductor manufacturing processes. While virtually all U.S. semiconductor manufacturing facilities utilize scrubbers or other related means for treatment of their effluent gases, that technology is not capable of removing all toxic or otherwise unacceptable impurities.
Although the levels of arsine, phosphine and other similarly toxic materials in typical process gas streams are relatively low, and can be further reduced by scrubbers, these levels are, under various state and federal standards, still above acceptable levels. On the other hand, if it were possible to effect substantially complete oxidation of these impurities, those oxidation products could be reduced to acceptable levels by conventional scrubber technology.
One solution to this problem, then, is to incinerate the process gas to oxidize the toxic materials, converting them to less toxic form. Conventional incinerators, however, typically achieve less than complete combustion. This is particularly so when the impurity to be incinerated is present in relatively low quantities. The problem is compounded when the process stream to be treated is composed primarily of a nonflammable gas such as nitrogen bearing the undesirable impurities. While some of the oxidizable impurities are removed by conventional incinerators, at least a portion of those impurities can escape the combustion zone, thereby creating at least the potential of toxic discharge into the atmosphere.
Another problem with utilization of conventional incinerators is the size of the incinerator and the temperature of the flue gas exiting the incinerator. Many semiconductor fabrication facilities have heretofore used scrubbers as the primary means for removing the impurities in question. In a large number of cases, recent more stringent air pollution regulations have necessitated a reduction in emissions. Even after an incinerator is installed, the scrubber will need to treat the flue gas of the incinerator to remove the oxides of the incinerated impurities. Thus, the incinerator will be installed, in many instances, as a retrofit device. The ducting and other process gas handling equipment in many existing facilities is not designed for high temperature gas, and flue gas from conventional incinerators can melt plastic ducting and/or create fire hazards.
Because retrofit applications are contemplated, the size of the incinerator is also a major concern. Desirably, the incinerator can fit and utilize existing ducting without requiring substantial additional space.
A further limitation of conventional incinerators is their lack of ability to both mix sufficient fuel with a nonflammable process stream in order to render it flammable and then to also achieve complete combustion of this fuel enriched gas. The choice of such fuel gas to be mixed with a nonflammable process gas is also important from the perspective of maintaining low operating costs, and the design must reflect this choice of fuel if proper burning characteristics are to be achieved.
Another problem with existing incinerators is their inability to handle the formation of solid oxides. The oxidation products of certain gaseous impurities in the waste gas from semiconductor and other processes are solids. For example, silanes oxidize to silica (sand). Accumulations of silica in most incinerators represents a substantial maintenance problem when those devices are used in the treatment of silane-containing gas streams.
In addition to converting toxic materials to their less toxic oxides, controlled incineration is desirable when the waste gas stream contains potentially flammable or explosive materials. Some semiconductor process gas streams, for example, contain substantial quantities of hydrogen gas or organic solvents, presenting obvious explosion hazards. Similarly, the presence of pyrophoric gases such as silanes can also present an explosion hazard. (Such pyrophoric materials have a tendency to spontaneously ignite in the presence of oxygen.)
An additional hazard potential exists when pipes leading to the incinerator contain premixed combustible mixtures which might be ignited by the incinerator flame and then propagate a flame backwards into the pipe with concomitant explosion potential. When conventional flame arrestors are used to prevent such flashbacks from occurring, they tend to plug up very quickly because of the solid oxide particles which are typically present in such mixtures, particularly those containing silane.
Accordingly, one object of the present invention is to provide an incinerator having increased ability to oxidize virtually all oxidizable components in a waste gas stream.
Another object of the present invention is to provide controlled combustion of flammable or explosive components of a waste gas.
Still another object of the present invention is to provide an incinerator that is compact and can merge with existing ducting.
Another object of the present invention is to provide an incinerator which converts a nonflammable gas stream into a flammable stream through the mixing in of additional fuel gas such that the entire mixture is burned and not merely heated in the incinerator.
A further object is to provide an incinerator which can effectively make use of inexpensive fuel in carrying out the conversion of nonflammable to flammable mixtures.
Yet another object is to provide an incinerator that has an effluent flue gas which is sufficiently cool to be compatible with existing ducting.
Another object is to provide an incinerator with improved capabilities for handling solid particulate combustion products.
An additional object of the present invention is the prevention of flashbacks without the associated plugging which typically occurs with use of conventional flame arrestors.
A further object is to reduce the tendency for all reactions to accelerate within the inlet pipe close to the flame by cooling the inlet section. Of particular interest among these reactions are those whose solid products of reaction tend to deposit in the pipes and clog them, as well as those reactions which accelerate flame speed as temperature is raised. One embodiment of the present invention employs active cooling of the inlet pipe(s).
Yet another object of the present invention is to increase the flow speed of gases introduced therein, such as nitrogen, which further reduces the likelihood of flashbacks by exceeding the normal flame propagation speed in the inlet section and by tending to dilute incoming combustible mixtures to below their flammability limits. Sufficient additional fuel to render the mixture flammable again may subsequently be added downstream.
The incinerator of the present invention is also able to burn oil mists that are generally non-combustible in other units, such as fluorinated oils used in vacuum pumps. Such vacuum pump oil mists tend to interact with silanes, halogens and other related compounds which are present in the pump exhaust. If the oil mists are not burned, these chemical interactions can lead to formation of hazardous solid materials in plant ductwork including cumulative deposition of certain explosive polymers. This problem is eliminated by the incinerator of the present invention, which is capable of burning such oil mists before they create a potential hazard.
Finally, another object of the present invention is to provide a unit with low maintenance requirements.
These and other objectives are fulfilled by the incinerator of the present invention.